Re: [GIT PULL] timer fixes

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>, x86-ml <x86@kernel.org>
Subject: Re: [GIT PULL] timer fixes
Date: Sat, 13 Aug 2022 13:27:40 -0700	[thread overview]
Message-ID: <CAHk-=wi+K-LQ121sPbsQonja3Sx-_kXZc6ntauUC2=JPsUrC+g@mail.gmail.com> (raw)
In-Reply-To: <YvfQUGLGY7cfZ9gf@zn.tnic>

On Sat, Aug 13, 2022 at 9:25 AM Borislav Petkov <bp@alien8.de> wrote:
>
> That task_struct.sighand is marked __rcu and thus noderef and sparse
> complains:

I think that RCU marking is misleading.

Doing a

        git grep -e '->sighand'

shows that we basically never treat that as some kind of RCU pointer.

Adding a

        grep -i rcu

to the above shows that we have a couple of places that do this
carefully, but they are the exception rather than the rule.

I think the issue is that "current->sighand" is always safe (and that
"me->sighand" is the same thing), and that sighand has RCU-delayed
freeing so that __lock_task_sighand() can safely try to take the lock
of another process' sighand.

And we have no real way to explain to sparse that *some* cases are
fine, others are not and need the sighand lock (after that careful
__lock_task_sighand thing).

              Linus